Most electronic circuits, such as integrated circuits, receive power from an externally-supplied power supply. For example, an electronic system may include a power supply (e.g., V33) that supplies power to one or more integrated circuits included in the system. At system start-up, V33 may start at an initial value (e.g., 0 volts), and then gradually increase to its full-scale value (e.g., 3.3 volts). Many integrated circuits, however, include chip configuration circuits or other circuits that require a minimum power supply voltage (e.g., 1.5 volts) for normal operation. If a power supply signal less than the minimum is applied to such configuration circuits, the chip may not operate properly. As a result, many integrated circuits use power-on reset (“POR”) circuitry to sense the voltage level of the power supply signal, and generate a control signal that indicates when V33 exceeds the minimum power supply voltage.
To accomplish this task, POR circuits typically compare the power supply signal with a reference signal that has a voltage level equal to the minimum power supply voltage, and generate a control signal that indicates when V33 is greater than the reference voltage. If the reference signal is an external signal (i.e., off-chip) that is always available, this task is quite straightforward. In most instances, however, an external reference signal is not available, but instead must be generated internally. Previously known POR circuits typically generate such reference signals by using properties of semiconductor devices, such as the threshold voltages of transistors and diodes.
For example, referring now to FIG. 1, a previously known POR circuit is described. POR circuit 10 includes trip detector circuit 12 and filtering circuit 14. Trip detector circuit 12 has an input coupled to V33, and generates an output signal XHI that may be used to indicate when V33 is greater than an internally-generated trip-point reference signal VREF. Filtering circuit 14 smoothes and further processes signal XHI, and generates an output control signal POROUT our that may be used to indicate when power supply signal V33 is sufficiently high for normal circuit operation.
Referring now to FIG. 2, an exemplary previously known trip detector circuit 12 is described. Trip detector circuit 12 includes diode-connected p-channel transistor 16 having its source terminal coupled to power supply V33. and its drain and gate terminals coupled together at node Vx. Node Vx also is coupled to ground via resistor 20, and to the gate of n-channel transistor 18. N-channel transistor 18 has its drain coupled to output node XHI, which also is coupled to power supply V33 via resistor 22. P-channel transistor 16 has a threshold voltage VTP having a nominal magnitude of about 0.8V, and n-channel transistor 18 has a threshold voltage VTN having a nominal value of about 0.8V. For simplicity, the symbol VTP will be used to refer to the magnitude of the threshold voltage of a p-channel transistor.
Referring now to FIGS. 2 and 3, the operation of exemplary trip detector circuit 12 is described. In particular, FIG. 3 illustrates V33, Vx and XHI as a function of time. At t=0, V33=0V, transistor 16 is OFF, and no current flows through resistor 20. As a result, Vx=0V, transistor 18 is OFF, no current flows through resistor 22, and XHI=V33=0V. For 0≦t<T1, V33 increases, but remains below VTP. As a result, transistor 16 remains OFF, and VX=0. At t=T1, V33 exceeds Vx by the threshold voltage VTP, and transistor 16 begins to conduct. If resistor 20 is very large, the drain current of transistor 16 is very small, and Vx remains one VTP below V33. For T1≦t<T2, the voltage on node VX increases with increasing V33, but remains below the threshold voltage VTN of transistor 18. Accordingly, transistor 18 remains OFF, no current flows through resistor 22, and thus XHI=V33. At t=T2, Vx is greater than VTN, and transistor 18 begins to conduct. If resistor 22 is large, the drain current of transistor 18 is small, and transistor 18 pulls XHI to ground. Thus, XHI changes from a positive non-zero voltage to 0V when V33 exceeds trip-point reference signal VREF=VTP+VTN.
Threshold voltages VTP and VTN, however, may vary significantly with variations in processing and temperature. For example, over normal process and temperature variations, threshold voltages VTP and VTN may have values between 0.6V to 1.2V. As a result, trip-point reference signal VREF may vary between VREFL=1.2V to VREFH=2.4V. For some circuit applications, such a wide variation in VREF may be unacceptable. For example, as described above, if a chip configuration circuit requires that V33 be at least 1.5V, such a circuit may fail if threshold voltages VTP and VTN are low (e.g., VTN=VTP=0.6V, and thus VREF=1.2V). Likewise, if threshold voltages VTP and VTN are both high (e.g., VTN=VTP=1.7V, and thus VREF=3.4V), XHI may never change state, and thus the POR circuit would fail.
In view of the foregoing, it would be desirable to provide methods and apparatus that reduce the sensitivity of trip point detection circuits to process and temperature variations.
It also would be desirable to provide methods and apparatus that increase the trip point reference VREF of trip point detection circuits when transistor threshold voltages are lowered as a result of process or temperature conditions.
It additionally would be desirable to provide methods and apparatus that decrease the trip point reference VREF of trip point detection circuits when transistor threshold voltages are raised as a result of process or temperature conditions.